FIG. 12 is a sectional view of a prior art gain guiding semiconductor laser. In FIG. 12, again guiding semiconductor laser 500 includes an n type InP substrate 1. There are successively disposed on the n type InP substrate 1 an n type InP lower cladding layer 2, an InGaAsP active layer 3, a p type InP upper cladding layer 5, and a p type InGaAsP contact layer 6. Reference numeral 13 designates a light emitting region of the active layer 3. An insulating film 7, such as SiO.sub.2, including a window opposite the light emitting region 13 is disposed on the InGaAsP contact layer 6. A p side electrode 8 is disposed on the insulating film 7, contacting the InGaAsP contact layer 6 through the window in that insulating film 7. An n side electrode 9 is disposed on the rear surface of the substrate 1. Reference numeral 10 designates a current injected into the laser structure from the p side electrode 8, numeral 11 designates effective current flowing into the light emitting region 13 of the active layer 3, and numeral 12 designates leakage current flowing outside of the light emitting region 13. The insulating film 7 is employed to concentrate the injected current toward the light emitting region 13 of the active layer 3.
A method for fabricating the gain guiding semiconductor laser 500 will be described.
Initially, the n type InP lower cladding layer 2, the InGaAsP active layer 3, the p type InP upper cladding layer 5, and the p type InGaAsP contact layer 6 are epitaxially grown on the n type InP substrate 1, preferably by MOCVD (Metal Organic Chemical Vapor Deposition). Thereafter, an insulating film is deposited on the contact layer 6 and patterned to form a window opposite a part to be the light emitting region 13 of the active layer 3. Thereafter, the p side electrode 8 is formed on the insulating film 7, contacting the contact layer 6 through the window of the insulating film 7, and the n side electrode 9 is formed on the rear surface of the substrate 1, completing the gain guiding semiconductor laser 500.
A description is given of the operation.
When a voltage is applied across the p side electrode 8 and the n side electrode 9, current flows from a part of the p side electrode 8 sandwiched by the insulating films 7 toward the n side electrode 9. More specifically, the injected current flows through the p type InGaAsP contact layer 6 and the p type InP upper cladding layer 5 while extending in the transverse direction in these layers and reaches the InGaAsP active layer 3. A part of that current flowing into the light emitting region 13, i.e., the effective current 11, causes laser oscillation.
In this prior art gain guiding semiconductor laser, however, all of the injected current 10 does not flow into the light emitting region 13 of the active layer 3. Some leakage current 12 flows outside of the light emitting region 13. Since the leakage current does not contribute to the laser oscillation, the light output of the laser is reduced. In addition, the magnitude of the leakage current 12 flowing through the active layer 3 varies when the thickness of the active layer 3 varies, and the magnitude of the effective current 11 flowing into the light emitting region varies because of the variation in the leakage current. Therefore, if the active layer 3 is not formed in a prescribed thickness in the production process, a semiconductor laser having desired operating characteristics is not achieved.
FIGS. 13 to 16 are sectional views of a prior art buried heterostructure semiconductor laser. In these figures, a buried heterostructure (hereinafter referred to as BH) semiconductor laser 600 includes a p type InP substrate 21, a p type InP lower cladding layer 22 disposed on part of the substrate 21, an InGaAsP active layer 23 disposed on the lower cladding layer 22, and a first n type InP upper cladding layer 24 disposed on the active layer 23. A stripe-shaped mesa structure 220 comprises the lower and upper cladding layers 22 and 24 and the active layer 23. A p type InP current blocking layer 26 is disposed on part of the substrate 21 and extending up the sides of the mesa structure 220 to the upper cladding layer 24. An n type InP current blocking layer 28 is disposed on parts of the p type InP layer 26. An additional p type InP current blocking layer 29 is disposed on the n type InP layer 28 and parts of the p type InP layer 26. A second n type upper cladding layer 30 is disposed on the p type InP current blocking layer 29 as well as on the top of the mesa structure 220, i.e., in contact with the first n type InP upper cladding layer 24. An n type InGaAsP contact layer 31 is disposed on the n type InP upper cladding layer 30. An n side electrode 9 is disposed on the contact layer 31, and a p side electrode 8 is disposed on the rear surface of the substrate 21. Reference numeral 10 designates a current injected into the laser structure from the p side electrode 8. Reference numeral 230 designates an effective current path. Reference numerals 231, 232, 233, and 234 designate first, second, third, and fourth leakage current paths, respectively.
A method for fabricating the BH semiconductor layer 600 will be described.
Initially, the p type InP cladding layer 22, the InGaAsP active layer 23, and the first n type InP upper cladding layer 24 are epitaxially grown on the p type InP substrate 21, preferably by MOCVD. Thereafter, the p type InP substrate 21 and those layers 22, 23, and 24 epitaxially grown on the substrate 21 are formed in a stripe-shaped mesa by conventional photolithography and etching techniques. Thereafter, the p type InP current blocking layer 26, the n type InP current blocking layer 28, and the additional p type InP current blocking layer 29 are successively grown on the substrate 21 contacting the opposite sides of the mesa structure. Preferably, these current blocking layers are grown by LPE (Liquid Phase Epitaxy). Thereafter, the second n type InP upper cladding layer 30 is epitaxially grown on the p type InP layer 29 and on the top of the mesa structure and, successively, the n type InGaAsP contact layer 31 is grown on the upper cladding layer 30. To complete the laser structure shown in FIG. 13, the p side electrode 8 is formed on the rear surface of the p type InP substrate 2 and the n side electrode 9 is formed on the n type InGaAsP contact layer 31.
A description is given of the operation.
When a voltage is applied across the p side electrode 8 and the n side electrode 9, current 10 flows between these electrodes. Most of the current flows along the current path 230, through the upper and lower cladding layers and the active layer, whereby laser oscillation occurs.
However, some leakage current flows along the path 231 through the p type InP substrate 21, the p type InP current blocking layer 26, and the n type InP upper cladding layer 24 (first leakage current path) as shown in FIG. 13 and along the path 232 through the p type InP substrate 21, the p type InP current blocking layer 26, the p type InP current blocking layer 29, and the n type InP upper cladding layer 30 (second leakage current path) as shown in FIG. 14. In other words, since these current paths 231 and 232 include forward-biased junctions, all of the current 10 injected into the laser structure does not flow into the active layer 23, i.e., some leakage current flows along these current paths 231 and 232. The leakage current does not contribute to the laser oscillation. Further, the p type InP current blocking layer 26, the n type InP current blocking layer 28, the p type InP current blocking layer 29, and the n type InP upper cladding layer 30 provide a pnpn thyristor structure, and very few charge carriers are present in the respective layers. Therefore, usually, leakage current hardly flows along the path 233 shown in FIG. 15 through those current blocking layers 26, 28, 29 and the upper cladding layer 30 (third leakage current path). However, when the thickness of the n type InP current blocking layer 28 is reduced in the production process for some reason or when the laser is operated at a high voltage, leakage current flows along the path 233. Further, when these current blocking layers are grown on opposite sides of the stripe-shaped mesa 220 by LPE, it is very difficult to control the thicknesses of these layers. The poor controllability of thicknesses sometimes causes unwanted contact between the n type InP current blocking layer 28 and the n type InP cladding layer 24 in the laser structure as shown in FIG. 16, producing a fourth leakage current path 234 through the p type InP substrate 21, the p type InP current blocking layer 26, the n type InP current blocking layer 28, and the n type InP cladding layer 24.
As described above, in the prior art BH semiconductor laser, some leakage current flows through the current blocking layers disposed at the opposite sides of the stripe-shaped mesa, whereby the light output of the laser is reduced. In addition, strain in the analog modulation cannot be reduced. Further, when the thicknesses of the current blocking layers vary, the magnitude of the leakage current flowing through the current blocking layers varies, whereby the magnitude of the effective current flowing into the active layer also varies. Therefore, if the current blocking layers are not formed in prescribed thicknesses in the production process, a semiconductor laser with desired operating characteristics cannot be achieved.
FIG. 17 is a sectional view illustrating a prior art BH semiconductor laser including a multiquantum barrier layer (hereinafter referred to as MQB layer) interposed between an active layer and a cladding layer. In the figure, a BH semiconductor laser 700 includes a p type InP substrate 1. An n type InP lower cladding layer 12a is disposed on the substrate 1. An InGaAsP active layer 23 is disposed on part of the lower cladding layer 12a. An MQB layer 25 comprising alternating InGaAsP barrier layers and InGaAsP well layers is disposed on the active layer 23. A p type InP upper cladding layer 22a is disposed on the MQB layer 25. A stripe-shaped mesa structure 220a includes the active layer 23, the MQB layer 25, and the upper cladding layer 22a. A p type InP current blocking layer 26 is disposed on part of the lower cladding layer 12a and extending up the sides of the mesa structure 220a to the upper cladding layer 22a. An n type InP current blocking layer 28 is disposed on the p type InP layer 26. An additional p type InP current blocking layer 29 is disposed on the n type InP layer 28. A p type InGaAsP contact layer 6 is disposed on the p type InP current blocking layer 29 as well as on the top of the mesa 220a, i.e., in contact with the p type InP upper cladding layer 22a. A p side electrode 8 is disposed on the contact layer 6 and an n side electrode 9 is disposed on the rear surface of the substrate 1. In this BH laser 700, the conductivity types of the respective layers are opposite to those of the BH laser 600 shown in FIG. 13.
The production process of this BH semiconductor laser 700 is identical to that already described with respect to the semiconductor laser 600 shown in FIG. 13 except that the MQB layer 25 is formed between the InGaAsP active layer 23 and the p type InP upper cladding layer 22a.
A description is given of the operation.
When a voltage is applied across the p side electrode 8 and the n side electrode 9, electrons and holes are injected into the active layer 23 from the n type InP lower cladding layer 12a and the p type InP upper cladding layer 22a, respectively. These electrons and holes recombine in the active layer to produce laser light, resulting in laser oscillation. During the laser oscillation, since the effective mass of electrons is several times smaller than the effective mass of holes, a part of electrons injected into the active layer 23 from the lower cladding layer 12a do not recombine with holes but flow toward the p type InP upper cladding layer 22a across the active layer 23. These leaking electrons reduce the luminous efficiency of the laser. In addition, the leakage of electrons is encouraged when the temperature in the vicinity of the active layer increases and the number of electrons having high potential energy increase. The MQB layer 25 reduces the unwanted leakage of electrons from the active layer 23 to the upper cladding layer 22a. That is, the MQB layer 25 provides a potential barrier having an effective energy larger than the band gap energy of the upper cladding layer 22a between the active layer 23' and the upper cladding layer 22a, whereby the leakage of electrons is reduced.
FIG. 18 is a sectional view of a part of the BH laser 700 shown in FIG. 17 in the vicinity of the active layer 23, showing electrons (.crclbar. ) and holes (.sym. ) during the laser oscillation. As shown in FIG. 18, although the MQB layer 25 reduces the leakage electrons from the active layer 23 to the p type upper cladding layer 22a, some electrons in the active layer 23 still leak to the p type current blocking layer 26. Therefore, even the MQB layer 25 does not adequately reduce the leakage of electrons so that the luminous efficiency of the output laser light can be increased to a satisfying level.
Furthermore, in the above-described prior art semiconductor laser devices 500, 600, and 700, since the leakage current increases as the injected current or the operating temperature increases, output characteristics and temperature characteristics of these prior art lasers are not improved.